ToonArmy,
NICE! It's looking great! I love the idea of it being inside like that, but to echo the above posts... you need to secure that baby down with a lots of rubble or some sandbags on the baseplate.... Something. As is, that'll skip all over you on that hard smooth clean floor with the first round out. Firing a mortar like that could very well ruin your day, not to mention the shot
NOT ending up on target.

A mortar puts out some serious recoil... even the smallest mortars. Hunker that baby down.
I'd say use lots of rubble (to add to the demolished building as well)... and throw in a few bags too.
Take a look at some of the 81mm mortars in position today. After the first round goes out, the baseplate is just about covered with and flush to the ground. Ya, maybe the crew stomped on it to seat (and level it) it, but the first round out usually will better seat the mortar for you. Also, if you'll note, WWII halftracks as well as the current day carriers need to have the floor strengthened to help absorb the recoil forces.
This all will all depend on the situation of course. Like mentioned above, if this is a hastily set up position, use what's available, but this would not be an 'ideal' firing platform as is.
You're on the right track with this ToonArmy! Just a little more time (and rubble) and it'll be
OUTSTANDING! 
I can't wait to see it completed!